Proteins are glycosylated in the endoplasmic reticulum. In the final structures, the glycan moieties are typically present:
Proteins are glycosylated in the endoplasmic reticulum. In the final structures, the glycan moieties are typically present:
Select an option to attempt
Correct Answer: Option A
Explanation
Location of glycan moieties on glycoproteins:Glycosylation occurs in the ER and Golgi. The key principle is topological:• Proteins enter the ER lumen (equivalent to the extracellular space topologically).• Glycosylation enzymes add sugars to the LUMENAL face of the ER/Golgi membrane.• As vesicles bud and fuse, the lumenal face always faces outward or into the lumen.• For secreted and plasma membrane proteins, glycans end up on the EXTRACELLULAR/OUTER surface.N-linked glycosylation: added to Asn in Asn-X-Ser/Thr motifs in the ER lumen.O-linked glycosylation: added to Ser/Thr in the Golgi lumen.Conclusion: Glycan moieties are present on the SURFACE (extracellular-facing side) of glycoproteins, never in the hydrophobic core or transmembrane segments.Answer: On the surface of proteins (Option 1).